Domestic hot water installations typically include a hot water tank which may be situated within a house or apartment or, alternatively, on the roof thereof. In either case, the hot water temperature is normally factory set, provision possibly being made for adjustment thereof during installation. Typically, such adjustment requires specialized knowledge and/or tooling and is beyond the ability of the average house-holder.
In such hot water systems, the water is maintained at the same constant temperature throughout both winter and summer. This is, clearly, an extravagant waste of energy since during the summer, when the ambient air temperature is high, a comfortable bath or shower can be enjoyed at a very much lower hot water temperature than during the winter months when the ambient air temperature is low. However, owing to the difficulty of adjusting the water temperature, the desired water temperature is factory set to a relatively high value such as 70.degree. C. suitable for winter use, whilst when the air temperature is 20.degree. or 25.degree. C., such as in hot summer conditions, a water temperature of 40.degree. C. would suffice.
In countries wherein the sun can be relied on to shine for most days of the year it is very common to provide hot water either exclusively or, at least in part, by solar heating. A solar panel is provided on the roof of the building and hot water is contained in a storage tank also normally situated on the roof of the building adjacent to the solar panel. Water is fed to the domestic hot water supply system via a thermally insulated pipe running from the storage tank on the roof of the building and gaining entrance to the domestic hot water supply.
Whether the water is heated using solar power or other means, some electrical backup is also normally provided in the form of an electrical immersion heater disposed within the hot water tank. The immersion heater brings the hot water up to temperature relatively quickly, and is often used fairly indiscriminately by a householder who wishes to take a bath or a shower quickly and speculates that the hot water temperature is inadequate. Owing to the fact that the householder has no way either of adjusting or even measuring the actual temperature of the hot water in the storage tank, the decision to boost the hot water temperature by using the electrical immersion heater amounts to guesswork on the part of the householder and, as often as not, will result in unnecessary wastage of energy.
Furthermore, owing to the high cost of electricity, it is obviously desirable to use an electrical emersion heater in order to boost the water temperature only to the extent that is absolutely required. For example, a person who wishes to boost the water temperature prior to taking a shower, clearly has no need to heat all of the water in the water tank which typically contains several hundred liters. Merely heating a fraction of the water tank's volume would suffice. However, it is no simple matter to indicate to the householder how much of a water tank's contents are at a desired, pre-set temperature. As is known, domestic water tanks allow for the hot water to be syphoned off at the top of the tank and to be replaced by cold water which is fed in at the bottom of the tank. Notwithstanding the constant hot water drainage and cold water replacement which thus ensues, there exists a marked temperature gradient throughout the tank such that the temperature of water toward the upper part of the tank is significantly greater than that of the water near the base of the tank. Thus, merely placing a temperature probe toward the upper surface of the tank would tend to provide an artificially high reading; whilst placing a temperature probe toward the base of the tank would provide an artificially low reading. In the former case, a householder might be tempted to take a shower under the misguided impression that there is sufficient hot water in the tank, only to find in practice that the water is too cold. In the latter case, the householder would be tempted to heat the water for an unnecessarily long time under the misguided impression that the water temperature has not yet reached a comfortable threshold.